Not applicable.
(1) Field of the Invention
The present invention relates generally to determining sound speed in water, and more particularly to a method and system for determining the sound speed profile of a water column from a moving or stationary position at the water""s surface or from some position under the surface.
(2) Description of the Prior Art
Knowledge of a water column""s sound velocity profile is useful in predicting performance characteristics of a sonar system. Currently, such sound velocity profiling is accomplished using expendable bathythermometric devices, towed cables or laser Doppler velocimeters. Bathythermometric devices measure water temperature and salinity in determining the speed of sound in water. However, thermistors used to measure temperature have fairly slow response times leading to inaccuracies in sound velocity determination. Towed resonant bubble cables or acoustic sensor cables are tethered to a towing vessel and, therefore, possess all of the drawbacks of towed systems to include their size, equipment needed to store and deploy the towed cable, and the inherent problems associated with towing something through the water. Laser Doppler velocimeters are complex devices requiring a substantial amount of equipment.
Accordingly, it is an object of the present invention to provide a method and system for determining the sound speed profile of a water column.
Another object of the present invention is to provide a method and system for simply and efficiently determining the sound speed profile of a water column by minimizing the amount of equipment required to do so.
Still another object of the present invention is to provide a method and system for determining the sound speed profile of a water column that does not require the towing or tethering of any equipment to a vessel.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a system and method are provided for determining a sound speed profile of a water column. A sound source is deployed in the water at a first known location at a specified time. Deployment can occur from onboard a moving or stationary watercraft. The sound source is free to fall through the water at a known rate of descent. The sound source houses an acoustic source for transmitting acoustic pulses omnidirectionally therefrom at predetermined times after the specified time at which the sound source was deployed. An acoustic receiver located at a second known location detects each acoustic pulse. A processor coupled to the acoustic receiver and programmed with the specified deployment time and predetermined transmit times determines a time differential between each predetermined transmit time and a time of arrival for each corresponding acoustic pulse. Speed of sound for each portion of the water column is then determined as a function of i) the time differential, ii) the first and second known locations, and iii) the known rate of descent of the sound source.